1. Technical Field of the Invention
The present invention relates to a production process of a polyphenylene ether composition that has a good light-discoloration resistance and causes little unmelted portion, and to a molded article formed from the polyphenylene ether composition.
2. Prior Art
A polyphenylene ether composition has heat resistance, which is arbitrarily determined according to an incorporation ratio of polyphenylene ether to polystyrene. The composition also has excellent properties such as insulation properties, dimensional stability, impact resistance, acid resistance, alkali resistance, low water absorbability, and low density. Further, it is good for the environment and excellent in safety and health because the flame retardancy can be imparted to the polyphenylene ether composition by incorporating a phosphorus or silicone compound instead of a halogen-type compound and antimony trioxide, which are assumed harmful. These excellent properties enable the polyethylene ether composition to apply to various uses such as electric and electronic components, office equipment parts, various kinds of housing materials, and industrial articles.
However, the polyphenylene ether composition has a drawback that is easy to discolor when exposed to light. In particular, it is disadvantageous in that it is likely to turn yellow with ultraviolet rays. Thus, its use is limited when it is colored to a chromatic color, especially a light color like white. Further, the polyphenylene ether composition does not exhibit sufficient releasability from a mold upon mold processing of a large-scale molded part, such as housing for office equipment like a printer, a fax machine, a copy machine, a multiple copy machine, or housing for a television.
In the past, various technologies were developed to improve the light-discoloration resistance of polyphenylene ether compositions. It has been commonly known to add an ultraviolet absorber such as a benzotriazole compound and a benzophenone compound and a hindered amine light stabilizer to the polyphenylene ether composition. Also, there has been disclosed the following technologies:
to use a hindered amine light stabilizer and a specific epoxy compound in combination (Patent Document 1);
to use a benzophenone compound, a hindered amine light stabilizer and a cyclic fatty acid epoxy compound in combination (Patent Document 2);
to use a specific benzotriazole compound and a hindered amine light stabilizer in combination (Patent Document 3);
to use a benzotriazole and an epoxy compound in combination (Patent Document 4); and
to use a mixture comprising 15 to 35 parts by mass of polyphenylene ether and 85 to 65 parts by mass of polystyrene, an ultraviolet absorber such as a benzotriazole compound and a benzophenone compound, a hindered amine light stabilizer, and a specific epoxy compound in combination (Patent Document 5).
However, no detailed definition to the production process has been given in these technologies to improve the light-discoloration resistance. Patent Document 6 discloses a production process of a polyphenylene ether composition, but not describe the light-discoloration resistance, and the process disclosed therein is not sufficient enough to achieve good light-discoloration resistance. The light-discoloration resistance of the resin composition is greatly influenced by a production process. For example, in the case that all raw materials are incorporated in one step and then melt compounded with a twin-screw extruder or the like, the extrusion temperature of polyphenylene ether is necessary to be preset at a high temperature. Especially, when a large amount of a liquid flame retardant is incorporated, the flame retardant is difficult to melt unless otherwise side-fed using liquid-feeding equipment. In this case, it is necessary to preset the extrusion temperature at 320° C. or higher. At such a high extrusion temperature, the temperature of the resin composition may reach 350° C. The light-discoloration agent volatilizes at such a high temperature of the resin composition, and the intrinsic light-discoloration resistance cannot be achieved.
When the extrusion temperature is adjusted to 300° C. or lower to prevent the volatilization, polyphenylene ether does not melt completely and an unmelted portion remains. As a result, problems are caused, such as deterioration of the impact resistance and defective appearance of the molded article, which occurs upon the molding processing. To melt polyphenylene ether completely, it is necessary to take some measures, such as reduction of the production rate. Also, the composition containing a smaller amount of polyphenylene ether hardly generates shear heat so that polyphenylene ether is harder to melt and an unmelted portion is more likely to remain. If a side-feeding location of the flame retardant is set downstream of the streamline in order to make polyphenylene ether easy to melt, the flame retardant may not be compounded with the resin completely. As the result, a liquid material spouts out of a dice part of the extruder, and stable production cannot be performed, owing to strand breakage or the like. Furthermore, there are problems in reduction of flame retardancy caused by the failure to incorporate the flame retardant in the prescribed amount. Therefore, there has been a demand for technologies that further improve the light-discoloration resistance.    [Patent Document 1] Japanese Patent Application Laid-Open No. S60-149646    [Patent Document 2] Japanese Patent Application Laid-Open No. S62-283183 (corresponding to U.S. Pat. Nos. 4,843,116, and 5,026,751)    [Patent Document 3] Japanese Patent Application Laid-Open No. H07-258514    [Patent Document 4] Japanese Patent Application Laid-Open No. H11-60934    [Patent Document 5] Japanese Patent Application Laid-Open No. H11-71488    [Patent Document 6] Japanese Patent Application Laid-Open No. 2002-541290 (corresponding to U.S. Pat. Nos. 6,258,879, and 6,486,244)